Laser with combined q-switch and synchronized cavity dump circuit

ABSTRACT

A high voltage electrical waveform generator circuit is provided which generates a voltage waveform having a positive d.c. voltage leading edge and a negative d.c. voltage trailing edge to the modulator. A gas trigger tube is provided in the circuit which generates a positive d.c. voltage leading edge in its plate circuit when it is independently triggered by a trigger signal. Another gas trigger tube generates a positive voltage when it is triggered by another trigger signal. A coupling circuit which couples the plate circuits of the trigger tubes includes capacitor which converts the positive voltage generated by the other trigger tube into the negative d.c. voltage trailing therefor edge of the voltage waveform. The coupling circuit may also include differentiator which allows the voltage levels on the trigger tube plates to be independently varied without affecting each other.

[ June 27, 1972 LASER WITH COMBINED Q-SWITCH AND SYNCHRONIZED CAVITY DUMP CIRCUIT Inventor: Ronald P. l'lllberg, Redondo Beach, Calif.

Assignee: TRW lnc., Redondo Beach, Calif.

Filed: Dec. 23, 1969 Appl. No.: 889,823

Related U.S. Application Data Division of Ser. No. 686,076, Nov. 28, 1967, Pat. No. 3,577,097.

U.S. CI ..328/210, 328/84, 315/229 Primary Examiner-Donald D. Forrer Assistant Examiner-B. P. Davis Att0mey-Daniel T. Anderson, Alfons Valukonis and Edwin A. Oser ABSTRACT A high voltage electrical waveform generator circuit is provided which generates a voltage wavefonn having a positive d.c. voltage leading edge and a negative d.c. voltage trailing edge to the modulator. A gas trigger tube is provided in the circuit which generates a positive d.c. voltage leading edge in its plate circuit when it is independently triggered by a trigger signal. Another gas trigger tube generates a positive voltage when it is triggered by another trigger signal. A coupling circuit which couples the plate circuits of the trigger tubes in- ..H0lj 17/02 Field of Search 328/84 210 315/229 cludes capacitor which converts the positive voltage 3011/1 6 generated by the other trigger tube into the negative d.c. voltage trailing therefor edge of the voltage waveform. The coupling circuit may also include differentiator which allows References cued the voltage levels on the trigger tube plates to be indepen- UNITED STATES PATENTS dently varied without affecting each other.

3,105,169 9/1963 Anderson et a1 ..328/84 X 6 Claims, 15 Drawing Figures 42 I I4 y |e l6 El "E i D= fl 10 34' TRIGGER Ll SH 36 GENERATOR fi-ME 93 ems Q-swn'cH DELAY 2/ lifii 38 PATENTEDaunzmzz 3 673 504 sum 10F a TRIGGER G EN ERATOR DELAY 22 CIRCUIT POCKELS 42 {CELL Cm 40pf Ace-aw 48 l PULSE e F a g. 2

RADIATION ENERGY INSIDE LASER CAVITY P'ATENTE'nJum m2 3. 673 504 sum 2 or s Q- SWITCH f ssl'zswr A Fig.4 a

PULSE 9 FROM I PHOTO DIODE to "I t VOLTAGE 9 ON f POCKELS cm.

" Fig.4c

l VOLTAGE 9 t I I w F |g.4d

' I ronsient l I 2 l-- VOLTAGE 9 QM ANODE |Tronsaem VOLTAGE ON ANODE 0F T4 f l 4 68 2 0 id Fig.7

P'A'TEN'TEDJUNZT I972 3.673.504

sum 30F a o-swncu sa a Mt iq PULSE 9 FROM PHOTO-DIODE 0 F|g.8b

VOLTAGE 9 ON v, POCKELS CELL a N Eng-8c VOLTAGE e, ON ELECTRODE 84 0F T3 Fig.8d

LASER WITH COMBINED Q-SWITCH AND SYNCHRONIZED CAVITY DUMP CIRCUIT CROSS REFERENCE TO A RELATED APPLICATION The present application is a Divisional Application of my prior co pending application Ser. No. 686,076 filed on Nov. 28, 1967, now U.S. Pat. No. 3,577,097.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to lwer apparatus and more particularly to laser apparatus having actuating circuitry for such apparatus which generates an actuating signal having combined Q-switching and synchronized cavity dumping signal portions to a single modulator such as a Pockels cell employed in the laser.

2. Description of the Prior Art In U. S. Pat. application Ser. No. 625,549, filed Mar. 23, 1967, and entitled Lasers Incorporating Time Variable Reflectivity, now U.S. Pat. No. 3,571,744 there is generally provided laser apparatus in which there is time variable reflectivity of the laser cavity during the lasing process. The time variable reflectivity relates to the variation of the reflectivity in the cavity during the power build-up prior to lasing and during lasing by varying the voltage at predetermined times to a polarity rotating means within the cavity and which acts to partially or totally effectively remove the reflectivity of one of the substantially 100 percent mirrors at the ends of the cavity. In the above-mentioned application a Q-switching operation is followed by a cavity-dumping operation. The sequence of events involved is to raise the excited atom population to the desired level while holding the radiation loss in the cavity high enough to prevent lasing, switch the loss in the cavity to a low value, thus allowing the optical energy in the cavity to build up to a maximum value, and to switch out the energy stored in the cavity. Successful operation of such apparatus depends on its ability to open the time variable mirror at the peak of the energy pulse. One embodiment of such apparatus employs a pair of modulators such as Kerr cells and active element actuating circuitry providing separate signals to the Kerr cells for Q- switching and synchronized vavity dumping.

Another U. S. Pat. application Ser. No. 686,267, filed Nov. 28, I967, entitled Self Synchronized Laser Apparatus and Method," reveals and describes laser. apparatus which also employs a pair of modulators with passive element actuating circuitry which generates separate signals for Q-switching and synchronized cavity dumping.

SUMMARY OF THE INVENTION In accordance with the present application there is provided a circuit for rapidly switching a high voltage and providing an output voltage wave with precisely determined leading and trailing edges. This circuit comprises a voltage storage capacitor having a first terminal adapted to be connected to a capacitive load. There is further provided a first and second gas switch tube, each having an anode. First circuit means are provided for coupling the first tube across the load. Furthermore, the first circuit means connects the anode of the first tube to the first terminal of the capacitor. A first dc voltage is provided which is connected by the first circuit means to the anode of the first tube. This will charge the first terminal of the capacitor to a first predetermined voltage. The first tube is responsive to a first trigger signal for discharging the first predetermined voltage of the capacitor at the first terminal.

Similarly, second circuit means are provided for coupling the second tube across the load. Furthermore, the anode of the second tube is coupled to the other terminal of the capacitor. A second dc voltage is provided which is connected to the anode of the second tube for applying a second predetermined voltage to the other terminal of the capacitor.

Accordingly, when a first trigger signal is applied to the first predetermined level. Similarly, application of the second trigger signal to the second tube will rapidly change the voltage across the load to a second voltage of opposite polarity to said first voltage.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic showing, partly in block diagram, of the laser apparatus incorporating the present invention;

FIG. 2 shows in schematic form the details of the preferred embodiment of the Q-switch and dump circuit of FIG. 1;

FIG. 3 shows a graph relating radiation energy occurring inside the laser cavity of FIG. 1 and time which is helpful to the understanding of the present invention;

FIGS. 4a 4e show graphs of voltages to be found in various parts of the circuit of FIG. 2;

FIG. 5 shows in schematic form the details of a modification of the circuit of FIG. 2;

FIG. 6 shows in schematic form the details of another embodiment of the Q-switch and dump circuit of FIG. 1;

FIG. 7 shows a graph of a voltage occurring in the circuit of FIG. 6; and

FIGS. 8a 8d show graphs of voltages to be found in various parts of a modification of the apparatus of FIG. 6 of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1 there is shown a laser apparatus 10 of the present invention for producing coherent radiation according to time variable reflectivity principles. The laser 10 generally comprises a pair of mirrors l4 and 16 defining a resonant cavity. Positioned within the cavity is a laser rod 17, a polarizer 18, and an electro-optic member 20. A flash lamp 21 is provided for pumping the rod 17. Positioned outside the cavity is an electric circuit generally designated by the numeral 22 for actuating the lamp 21 and the electro-optic member 20.

The rod 17 can be comprised of neodymium. The opposite ends of the rod 17 are directed toward the reflecting surfaces of the mirrors l4 and 16 with the rod having its longitudinal axis perpendicular to the mirrors. For regeneration of the excited atoms in the laser rod 17, the output beam therefrom is confined along its longitudinal axis and is polarized in a direction normal to the axis of the rod 17. The direction of polarization of the laser radiation is determined by the orientation of the polarizer 18. For Glan polarizer 18 shown in FIG. 1, the direction of the polarization of the laser radiation is horizontal, or in the plane of the drawing. Adjacent to the rod is positioned the flash lamp 21 which can be a xenon flash tube, for example.

The mirrors 14 and 16 are positioned parallel to each other and are substantially 100 percent reflective. The polarizer 18 and the electro-optic member 20 are in axial alignment with the laser rod 17. The laser cavity between the mirrors 14 and 16 has a length of approximately 24 inches. All of the flat surfaces within the cavity except the mirrors can be anti-reflection coated.

The polarizer 18 acts to send radiation in a particular direction, depending on the polarity direction of the radiation. The polarizer 18 may be a Glan polarizer and fashioned from a material such as calcite. The polarizer has a shield 30 on the side which prevents the emission of light from that side.

Radiation which is coming toward the polarizer 18 from the left or in other words from the direction of the laser rod 17, passes undiverted through the polarizer if such radiation is horizontally polarized, but is diverted into the absorber 30 if such radiation is vertically polarized. Radiation which is approaching the polarizer 18 from the right, or in other words from the direction of the mirror 16, also passes undeviated through the polarizer 18 if such radiation is horizontally polarized, but is diverted in the output direction 32 if such tube, a first voltage across the load is rapidly reduced to a first radiation is vertically polarized.

The electro-optic member acts as a polarization rotator, when energized, and is efiective to rotate the direction of the polarization of the polarized radiation through predetermined angles depending upon the voltage applied. When it is not energized it does not affect the polarity of the laser radiation. The electro-optic member 20 may be a Pockels cell and is in axial alignment with both the rod 17 and the polarizer 18. The Pockels cell 20 may be of the type commercially designated as BOA-415B and which can be activated with a voltage pulse of 6,000 volts at 1.06 microns. Thus, when there is no voltage on the Pockels cell 20 an undeviated transmission of laser radiation takes place through the Pockels cell 20 and the polarizer 18. When the Pockels cell 20 is energized, the path of the polarized laser radiation is controlled by the rotation at the Pockels cell 20 such that it is diverted by polarizer 18 in the output direction 32. The Pockels cell 20 may be energized with either a positive voltage or a negative voltage, and the effect of the Pockels cell 20 on the polarized laser radiation is the same. In other words, the laser radiation can be diverted by polarizer 18 in the output direction 32 by the application of either a positive 6,000 volts or a negative 6,000 volts to the Pockels cell 20.

In accordance withthe time variable reflectivity principles of this invention there occurs a high cavity loss interval when the Pockels cell 20 is positively biased, a laser radiation buildup interval when the Pockels cell 20 is unbiased, and an output or cavity dumping interval with the Pockels cell 20 negatively biased.

The circuit 22 consists of a trigger generator 34 which provides an output signal e for actuating a flash lamp driver 36 to pump the lamp 21 to excite the atoms in rod 17. A delay circuit 38 connected to the trigger generator 34 and a Pockels cell Q-switch and cavity dump circuit 40 serves to delay the signal e, a predetermined time so as to provide the circuit 40 with a delayed signal 2,. A photodiode 42 positioned outside the laser serves to monitor the light output leaking through mirror 14 and generates a signal 2 to the circuit 40. The circuit 40 is responsive to the delayed signal e and the signal e and is connected to the Pockels cell 20 so as to provide a bias signal e in its output thereto.

Reference is now made to FIG. 2 wherein there is shown the details of the circuit 40 of FIG. 1 which is capable of generating the voltage waveform e needed to accomplish the necessary sequence of events, namely a high cavity loss interval, a laser radiation build-up interval and a cavity dumping interval. A trigger gas tube T1 is adapted to have its control grid 48 connected to the delay circuit 38 for actuation thereby with the delayed signal e,. A resistor 49 connects the grid 48 to ground. The anode 50 of tube T1 is connected through a resister 52 to a source (not shown) of variable d.c. voltage V,. The tube T1 may be a Krytron cold cathode switch tube type KN-22. The anode 50 of tube T1 is also connected through a resistor 58 and coaxial cable 60 to the Pockels cell 20. A capacitor 62 along with the resistor 64 serves to form a differentiator circuit 66, which is A.C. coupled by capacitor 68 to the plate 70 of a gas switch tube T2. The tube T2 can be a type KN-6B Krytron tube which is commercially available. The anode 70 is connected through a resistor 72 to a variable dc. voltage V supply source (not shown). A keep-alive grid 74 of tube T2 is likewise connected through a resistor 76 to the source of variable dc. voltage V The control grid 78 of tube T2 is connected through a capacitor 80 to the photodiode 42. A resistor 82, connected at one end to the side of the capacitor 80 which is connected to photodiode 42, is connected at the other end to a source (not shown) providing a 4 Kv. dc. voltage. A resistor 83 connected at one end to the control grid 79 is connected at the other end to ground.

Operation of the laser apparatus of this invention can best be described with reference to FIGS. 1, 2, 3, and 4. Initially the voltages V, and V are each independently adjusted to 6 Kv., for example, and are applied to the plates of tubes T1 and T2, respectively. The application of a voltage V on the plate of tube Tl provides a positive dc voltage on the Pockels cell which causes rotation of the polarization of radiation emanating from the rod 17 such that a high energy loss condition is established within the laser cavity. In this condition laser radiation emanating from the rod 17 is diverted by polarizer 18 either into the absorber 30 or in the output direction 32, depending on the polarization of the radiation. Next, at time t, the trigger generator 34 is actuated to provide the signal e, which causes the flash lamp driver 36 to energize the lamp 21 to start nonregenerative pumping or irradiating of the atoms in the rod 17 The number of excited atoms in the rod 17 reaches a maximum within a few hundred microseconds. The signal 2 is delayed by the delay circuit 38 for a time (t t which is equal to the time it takes for the number of excited atoms to reach a maximum, and at time t as shown in FIG. 4a, the delayed signal :2 is applied to the grid 48 of tube Tl causing tube T1 to conduct heavily, and therefore causing the anode 50 thereof to short to ground and the voltage e on the Pockels cell 20 also to go quickly from V to zero. With voltage V thus removed from the plate 50 of tube T1 and the Pockels cell 20, an undeviated transmission of radiation takes place through the polarizer 18 and the Pockels cell to the mirrors l4 and 16 such that lasing begins to take place within the cavity, and the radiation energy starts to build up within a frequency range including the characteristic frequency of the lasing atoms in the rod 21.

The change from V to zero in voltage e which occurs at time t. is differentiated by the differentiator network 66, and thus appears as a short negative going transient in the voltage 2 as shown in FIG. 4d. A similar negative transient appears in the voltage e on the anode of tube T2 as shown in FIG. 4e. The purpose of the differentiating circuit 66 is to prevent the entire step change in the voltage e from being AC coupled to the anode of T2, to thus subtract from the voltage level V whichnormally appears on the anode of T2. The use of the differentiating circuit 66 thus prevents interacn'on between the changes in the hold-off voltage V and the dumping voltage V An interval of about nanosecond occurs before the radiation energy reaches a significant level as indicated at t, in FIG. 3 after which the level rapidly rises to its peak intensity designated at t,,. That is, after time t photons are emitted from the laser rod 21 and make many traversals within the cavity through the rod 17 and between the mirrors I4, 16, passing through the Pockels cell 20 and the polarizer 18 without deviation. As the radiation energy builds up between t., and r in FIG. 3, the population of excited atoms rapidly decreases. The time between t, and t,,, is approximately 200 nanoseconds. The power leaking through the mirror 14 builds up at the same rate as the energy within the cavity, and thus the voltage pulse that appears at the output of photodiode 42 has the same shape as that of the energy pulses of FIG. 3. This voltage pulse is shown in FIG. 4b and is designated e,.

The pulse e is applied to the trigger grid 78 of tube T2 whereby the tube T2 becomes shorted to ground and the voltage on its plate 70 drops to zero as shown in FIG. 4e, and the voltages e and e. develop negative-going portions as shown in the FIGS. 4c and 4d, respectively. There is an internal delay in the tube T2, such that a short interval of time elapses between the application of a triggering voltage to its grid 78 and the time at which the tube T2 becomes conducting. This internal delay can be changed by changing the keep-alive current applied to the grid 74. In the operation of this embodiment of the invention the amount of keep-alive current and the amplitude of the signal e applied to the grid 78 of tube T2 are adjusted such that tube T2 becomes conducting at the time 1,, when the laser radiation energy within the cavity is at a maximum. With the application of the negative-going portion of the 2 voltage to the Pockels cell 20 at time t,, as shown in FIG. 4c, rotation of the polarization of the radiation in the cavity is achieved and the energy is caused to follow the path 32 out of the polarizer 18. It will be appreciated that it is only necessary that the voltage be maintained at the negative value for a few nanoseconds, since its only function is to cause draining of the laser cavity of radiation.

The circuit of FIG. 2 can be modified to decrease the fall time of the negative bias voltage portion of the voltage e used to dump the cavity at time t,,. In this modification a type KN-22 tube is substituted for the KN-6B tube utilized in the embodiment of FIG. 2. The KN-22 has a faster tum-on time than the KN-6B, but has a lower voltage capability. Thus, in order to achieve the desired magnitude of the negative bias voltage portion of the voltage e the circuit of FIG. 2 is also provided with a voltage doubler in the form of a length of coaxial cable 67 such as RG-58, 50 ohm cable which is inserted between the capacitor 68 and the junction of capacitor 62 and a resistor 69 having an ohmic value of 500, substituted for the resistor 64, as shown in FIG. 5. The length of cable 67 should be such as to provide voltage doubling. The operation of this modified circuit is the same as that of the circuit of FIG. 2 except that a lower voltage V, can be applied to the anode of T 7 Reference is now made to FIG. 6 wherein there is shown the details of still another embodiment of the Q-switch and dump circuit of FIG. 1. The circuit of FIG. 6 is somewhat similar to the circuit of FIG. 2 and like numerals designate like parts. In FIG. 6 a triggered spark gap tube T3 and a gas switch tube T4 with associated components are substituted for the tube T2 of FIG. 2. The tube T3 can be a commercially available GP-l7A and the tube T4 can be a KN-22. The opposite electrode 84 of the tube T3 is adapted to be connected through a resistor 86 to the source (not shown) of variable d.c. voltage V The adjacent electrode 88 of the tube T3, is connected to ground and the trigger probe 90 is connected through a resistor 92, coaxial cable 94, capacitor 96 to the plate 98 of tube T4. A resistor 100 connects the connecting point between the resistor 92 and cable 94 to ground. The control grid 102 of tube T4 is connected as shown and the keep-alive grid 104 is connected through a resistor 106 to a source (not shown) of d.c. voltage V;,. A resistor 108 connects the anode 98 to the same source (not shown) supplying the d.c. voltage V The resistor 82 is connected to a source (not shown) supplying a 2.5 Kv d.c. voltage. The cable 94 serves as a voltage doubler and is long enough to achieve the desired voltage doubling.

Operation of the circuit of FIG. 6 is similar to the operation of the circuit of FIG. 2 up through the time t, and can best be described having reference to FIGS. 3, 4, 6 and 7. The voltage V V and V, are initially applied to the tubes T1, T3, and T4, respectively. As before, the application of a voltage V, on the plate of tube T1 provides a positive d.c. q bias on the Pockels cell 20 which causes radiation emanating from the rod 17 to be diverted either into the absorber 30 or in the output direction 32, depending on the polarization of the radiation to establish a high energy loss condition within the laser cavity. At the time 1,, the trigger generator is actuated to provide the signal e to the lamp driver 36 which energizes the lamp 21 to start the pumping of atoms in the rod. The signal e is delayed for the time (t, t and is then applied to the grid 48 of tube Tl causing it to conduct and the voltage e on the Pockels cell 20 to go from a voltage V to zero. With the voltage V removed from tube T1 and the Pockels cell 20 lasing begins within the cavity as before. The change in voltage e occurring at time t,, appears as the negative going transient in the voltage 2 and as a similar transient in the voltage e appearing on the electrode 84 of tube T3. The circuit of FIG. 6 can be triggered at a somewhat lower voltage level from the phototube 42 and has a shorter trigger delay than that inherent in the tube T2 of FIG. 2. As the energy in the laser cavity builds up, some of this energy leaks through the mirror 14 and, as in the embodiment of FIG. 2. energizes the photodiode 42 to produce the cavity dump signal e,. The pulse e, is applied to the grid 102 of the tube T4 whereby the tube T4 becomes shorted to ground and the voltage V; on the anode of T4, as shown in FIG. 7, drops to zero producing a negative going pulse which is doubled in magnitude by the voltage doubler cable 94. This negative going pulse is a.c. coupled to the trigger probe 90 of tube T3 which becomes shorted to ground and the voltage on its opposite electrode 84 drops to zero as shown in FIG. 4e. This negative going voltage is a.c. coupled back to the Pockels cell 20, and the voltage thereon goes from zero to a negative value, as shown in FIG. 4c.

The circuit of FIG. 6 can be modified to eliminate the differentiator circuit 66 therefrom. Operation of such a modified circuit is similar to the operation of the circuit of FIG. 6 and provides the corresponding voltage waveforms shown in FIGS. 8a-8d. With this circuit a shorter time interval between 0- switching time t, and cavity dumping time t, can be obtained, since it is not necessary to wait for the differentiated pulse a, of the circuit of FIG. 6 to recover. However, a change in either one of the applied voltages V or V, afiects both the Q- switching and dumping voltage portion levels applied to the Pockels cell.

It will be obvious to those skilled in the art that many other changes and modifications of the invention may be made without departing from the trupe scope thereof as defined in the appended claims.

I claim:

1. A circuit for rapidly switching a high voltage and providing an output voltage wave with precisely determined leading and trailing edges, said circuit comprising:

a voltage storage capacitor having a first terminal adapted to be connected to capacitive load;

a first trigger gas switch tube having an anode;

first circuit means for coupling said first tube across the load and for connecting the anode of said first tube to said first terminal of said capacitor;

a first source of direct-current voltage connected by said first circuit means to the anode of said first tube for charging said capacitor to a first predetermined voltage, said first tube being responsive to a first trigger signal for discharging said first predetermined voltage of said capacitor at said first terminal;

a second trigger gas switch tube having an anode;

second circuit means for coupling said second tube across the load and the anode of said second tube to the other terminal of said capacitor; and

a second source of direct-current voltage connected by said second circuit means to the anode of said second tube for applying a second predetemrined voltage to said other temiinal of said capacitor, said second tube being responsive to a second trigger signal for discharging said second predetermined voltage at said other terminal of said capacitor, whereby when a first trigger signal is applied to said first tube a first voltage across the load is rapidly reduced to a first predetermined level, and whereby application of the second trigger signal to said second tube will rapidly change the voltage across the load to a second voltage of opposite polarity to said first voltage.

2. A circuit as defined in claim 1 wherein a differentiator circuit is coupled between the anode of said second tube and the other terminal of said capacitor for preventing changes of said first voltage from affecting said second predetermined voltage.

3. A circuit as defined in claim 1, wherein a voltage doubler circuit is coupled between the anode of said second tube and the other terminal of said capacitor for increasing the magnitude of said second predetermined voltage.

4. A circuit as defined in claim 3, wherein said voltage doubier circuit includes a predetermined length of coaxial cable.

5. In combination:

a first trigger gas switch tube having an anode adapted to be connected to a first source of variable d.c. voltage supplying a first predetermined voltage at the anode, said first tube being responsive to a trigger signal for eliminating said voltage at the anode;

a second trigger gas switch tube having an anode connected to a second source of variable d.c. voltage supplying a second predetermined voltage at the second tube anode, said second tube being responsive to a trigger signal for eliminating said second voltage at the second tube anode; and

to a second source of variable d.c. voltage supplying a second predetermined voltage at the second tube anode, said second tube being responsive to a trigger signal for eliminating said second voltage at the second tube anode; and

circuit means interconnecting said anodes, said circuit means comprising a capacitor, a coaxial cable, and a differentiator arranged in series connection. 

1. A circuit for rapidly switching a high voltage and providing an output voltage wave with precisely determined leading and trailing edges, said circuit comprising: a voltage storage capacitor having a first terminal adapted to be connected to capacitive load; a first trigger gas switch tube having an anode; first circuit means for coupling said first tube across the load and for connecting the anode of said first tube to said first terminal of said capacitor; a first source of direct-current voltage connected by said first circuit means to the anode of said first tube for charging said capacitor to a first predetermined voltage, said first tube being responsive to a first trigger signal for discharging said first predetermined voltage of said capacitor at said first terminal; a second trigger gas switch tube having an anode; second circuit means for coupling said second tube across the load and the anode of said second tube to the other terminal of said capacitor; and a second source of direct-current voltage connected by said second circuit means to the anode of said second tube for applying a second predetermined voltage to said other terminal of said capacitor, said second tube being responsive to a second trigger signal for discharging said second predetermined voltage at said other terminal of said capacitor, whereby when a first trigger signal is applied to said first tube a first voltage across the load is rapidly reduced to a first predetermined level, and whereby application of the second trigger signal to said second tube will rapidly change the voltage across the load to a second voltage of opposite polarity to said first voltage.
 2. A circuit as defined in claim 1 wherein a differentiator circuit is coupled between the anode of said second tube and the other terminal of said capacitor for preventing changes of said first voltage from affecting said second predetermined voltage.
 3. A circuit as defined in claim 1, wherein a voltage doubler circuit is coupled between the anode of said second tube and the other terminal of said capacitor for increasing the magnitude of said second predetermined voltage.
 4. A circuit as defined in claim 3, wherein said voltage doubler circuit includes a predetermined length of coaxial cable.
 5. In combination: a first trigger gas switch tube having an anode adapted to be connected to a first source of variable d.c. voltage supplying a first predetermined voltage at the anode, said first tube being responsive to a trigger signal for eliminating said voltage at the anode; a second trigger gas switch tube having an anode connected to a second source of variable d.c. voltage supplying a second predetermined voltage at the second tube anode, said second tube being responsive to a trigger signal for eliminating said second voltage at the second tube anode; and circuit means interconnecting said anodes, said circuit means comprising a capacitor and a differentiator arranged in series connection.
 6. In combination: a first trigger gas switch tube having an anode adapted to be connected to a first source of variable d.c. voltage supplying a first predetermIned voltage at the anode, said first tube being responsive to a trigger signal for eliminating said voltage at the anode; a second trigger gas switch tube having an anode connected to a second source of variable d.c. voltage supplying a second predetermined voltage at the second tube anode, said second tube being responsive to a trigger signal for eliminating said second voltage at the second tube anode; and circuit means interconnecting said anodes, said circuit means comprising a capacitor, a coaxial cable, and a differentiator arranged in series connection. 